System for measuring phase distortion in transmission networks, particularly cables



March 24, 1953 J. sELZ 2,632,792

SYSTEM FOR MEASURING PHASE DISTORTION IN TRANSMISSION NETWORKS, PARTICULARLY CABLES Filed Feb. 26, 1948 2 SHEETS- SHEET l Hg. 5 I/ 65 Fo F 643 A F .i Fed-M J6 (QZ March 24, 1953 J. sELz f 2,632,792

SYSTEM EOR MEASURING PHASE OISTORTION IN TRANSMISSION NETWORKS, PARTIOULARLY CABLES Filed Feb. 26, 1948 2 SHEETS-SHT 2 HMI? 19a/A55 7H/Frm@ 75 weg Ffa/V1 /4 15' b2@ 22 Patented Mar. 24, 1953 SYSTEM FOR MEASURING PHASE DISTUR- TIGN IN TRANSMISSION NETWORKS, PAR- TICULARLY CABLES Jacques Selz, Paris, France, assignor to Compagnie Industrielle des Telephones, Paris,

France, `a corporation of France Application February 26, 1948, Serial No. 11,176 In France January 31, 1947 Section 1, Public Law 690, August 8, 1946 Patent expires January 31, 1967 6 Claims.

This invention relates to systems for use in the measurement of delay distortion as represented by change in phase and attenuation distortion in four-terminal transmission networks, particularly cables, and comprises an improved system for comparing the propagation times of waves of different frequencies for the purpose of determining the delay distortion of the channel or system and adapted also, if desired, to measure the relative attenuation at different frequencies for the purpose of determining the attenuation distortion of the system over a required range of frequencies.

It is well known in the telecommunication art that the propagation time through a given channel under consideration for a group of waves of any given frequency f can be determined by transmitting an amplitude-modulated wave whose carrier frequency is f and which is modulated at a modulating frequency F, through the system under test and measuring the difference of 'phase A@ of the modulating wave of frequency F at the transmitting and receiving ends of the channel or system, the modulating frequency F being small compared with the carrier frequency j'. The ratio of the differentials (where w=2vrf) is equal to the propagation time T of the channel or system for a wave of the carrier frequency f. If the modulating frequency F is small enough a mean value fm of the propagation 'time through the channel over a range of frequencies extending from f-F, to f|F sufficiently accurate for practical purposes is obtained according to the equation The system according to the present invention is arranged to transmit two'amplitude modulated waves of different carrier frequencies fn and f through a four terminal network under test, the two carrier waves being respectively modulated with two modulating frequencies Fo and F one of which is a multiple of the other, and comprises means for demodulating the waves at the output end of the test network and for separating the two modulating frequencies F and F thus obtained after demodulation, a frequency multiplier arranged to convert the lower frequency F0 so obtained to the higher frequency F, and means for measuring the difference of phase between ithe waves of equal frequencythus ob- (Cl. F- 183) tained. From the difference of phase u thus measured, the difference between the transmission time rm through the test channel for a wave of any carrier frequency f and the transmission time vmo for a wave of reference frequency fo can be determined according to the equation:

To ensure accuracy in the phase relation between the two modulating frequencies Fo and F at the transmitting end, these two frequencies are preferably obtained from a single generator, one of the frequencies being Iderived from the generator generating the carrier frequency by means of a fequency-multiplier.

The system may be adapted for measuring differences of attenuation at different frequencies by transmitting two amplitude-modulated waves of different carrier frequencies through the system under test, means being provided for demodulating the waves at the receiving end and for separating the reconstituted modulation frequencies and measuring their relative amplitude.

The nature of the invention and of the features thereof and the manner in which the same is to be performed will be understood from the following detailed description, given by way of example, of systems arranged according to the present invention, reference being made to the accompanying drawings in which:

Figure l is a schematic .diagram of an arrangevment for measuring the delay distortion of a transmission cable,

Figure 2 is a diagram of a first phase-difference measuring apparatus for use in the arrangement of Fig. 1,

Figure 3 is a diagram of a modified arrangement of phase-dierence measuring apparatus usinga cathode ray tube for use in the arrangement of Fig. 1,

Figure 4 is a schematic diagram of a simplified installation for measuring differences of attenuation, and

Figure 5 shows a differential detector suitable for use in the` system shown in Figure 4.

n In Figure l, the circuit element I is a low frequency oscillator adapted to produce a modulating alternating current of frequency Fo, which is used' in the modulator 3- to amplitude-modulate a high frequency oscillation of fixed frequency ,fo generated by a high frequency oscillator 2. The modulating frequency Fo obtained from the oscillator i is also applied to frequency multiplier 4 by which a frequency FznFo is obtained. This current of frequency F is so obtained through the medium of a phase correcting device 5 to a modulator 'I in which it is used to amplitudemodulate an oscillation of variable frequency f produced by a high frequency generator 6. The oscillators, modulators, multipliers, and other units just mentioned are all at the input end of the cable or network under test.

The two modulated waves are mixed in a mixing stage 8 and applied to the input terminals of a four-terminal network 9 to be tested.

The output terminal circuit I of the quadripoles or network 9 is immediately followed by a non-selective detector II which is connected to two band filters I2 and I3 in parallel designed to separate the so detected two modulation frequencies F and F0. The frequency Fo of the current which is received by the filter I2, is multiplied by the factor n by means of a frequency-multiplier I i so that two waves of the same frequency are obtained at the outputs of the elements I3 and I i respectively. These two currents of phase and frequencies are relatively displaced in phase by the phase angle, ,//=Afp-A po. This difference of phase is measured by a phase-meter I5.

Generally the modulator 'I and the mixer 8 will introduce group-frequency transmission delays which are Variable as a function of the variable frequency f of oscillator 6. To eliminate this cause of error, the phase-corrector 5 is arranged to effect a phase correction such as to maintain a constant phase relation between the waves applied to the transmitting end of the test, quadripole or network 9. This constant phase relation is checked by means of a monitor system consisting of a detector l I', filters I2 and I3', a frequency-multiplier Ill' and a phasemeter i5', which correspond to the circuit elements II, I2, I3, I4 and I5 respectively and which operate in the same way.

Figure 2 illustrates the preferred arrangement of the phase-meter I5. It comprises a pair of amplifiers E6 and I'I arranged to receive the outputs o-f frequency F at the phase angles as mentioned from the frequency-multiplier i4 and lter I3 respectively. The output from the amplier I6 passes through an attenuating device I3 and a calibrated phase-shifting device 2i! while the output from the amplifier I'I passes through an attenuating device I9 and a calibrated phaseshifting device 2l. The outputs from the phaseshifting devices 23 and 2i are united in a mixer 2.2; the mixed wave is demodulated by a detector 23, and the demodulated output is applied to an indicating instrument 24 such as a galvanometer. As adjustment is made of the attenuating devices I8 and i9 and the calibrated phase-shifting devices 2G and 2I so that the instrument 26 gives a null indication; and the resultant change of phase angle gb can then readily be deduced 4from the readings of the phase-shifting devices 20 and 2I after such adjustment.

Figure 3 represents another example of a phasemeasuring apparatus I5 in which an oscillograph is used. In this arrangement of the outputs of frequency F at the phase angles as mentioned from the filterIB and frequency-multiplier Id respectively are passed through ampliers 25 and 26 respectively to two adjustable attenuating devices 21 and 28 and thence through two calibrated phase-shifting devices 23 and 3B. The output from one of the phase-shifting devices 29 and 39 is applied to the horizontal deiiecting plates of a cathode-ray tube 3i and the output from the other phase-shifting device is applied to the vertical deflecting plates of the cathode-ray tube. When these two outputs have been brought into phase or into phase opposition by adjustment of the phase-shifting devices 29 and 39, the trace on the screen of the cathoderay tube3| takes the form of a straight line and the settings of the phase-shifting devices are thend read to determine the change of phase which has resulted. This arrangement is more` rapid in use than the null indicating arrangement described with reference to Figure 2 because it is not necessary to obtain exact equality of amplitude between the outputs obtained from the two phase-shifting devices 29 and 30. However, owing to the size of the spot of the cathode ray tube, it is not possible with this arrangement to observe small differences o-f phase less than say one degree.

The invention includes an arrangement for increasing the accuracy of measurement by multiplying the frequencies F by the same factor p by means of frequency-multipliers. In Figure 3, the rectangles 32 and 33 indicate frequencymultipliers inserted between the phase-shifting devices 29 and 3i) and the input terminals of the oscillograph 3I for the purpose of increasing the accuracy of the results obtainable. If the frequency-multiplying factor p is 5, for example, the phase diiferencefat the frequency F can be measured to within 0.2 degree if the difference of phase at the frequency pf can be read to an accuracy of within one degree.

Referring to Figures 1 and 2, it will be observed that the attenuating devices I8 and I9 can be used to compare the attenuations of the reference frequency and variable frequency waves fo and f respectively by noting the adjustments of the attenuating devices necessary to obtain a null indication in the instrument 24. n order to obtain an effective measurement in this way, it is essential toensure that the mean amplitude and modulation factor of the variable frequency wave applied to the transmitting end of the network 9 is constant and that the attenuation due to the phase-adjusting devices used does not vary with adjustment of the latter. These requirements can readily be satised by methods known in the art.

Figure 4 shows an example of a system according to the invention arranged to measure differences of attenuation without regard to the differences in the propagation times of the wave envelopes. The apparatus is simplified as compared with that shown in Figures l and 2, as the frequencies F and F0 do not have to be multiples of one another. In Figure 4 the circuit element 34 is a low-frequencyv oscillator which generates the frequency Fo applied to a modulator 36 by which a xed frequency fo generated by a highfrequency oscillator 35 is modulated. The circuit element 37 is another high-frequency oscillator for producing a variable frequency f which is modulated in a modulator 39 by a modulating frequency F produced by a generator 38. By means of a mixer t0, the two modulated waves are applied to the transmitting end of a network I which terminates in a circuit 42. A detector 43 and a pair of filters 44 and 45 are arranged to detect the two carrier frequencies Fo and F and to separate them. The output from the lter 4d is passed through an amplier 46 53? and attenuatigldevic 48 for thetfrequency Fo and the. output from tlfelter 45 ispassed through anamplifier 41 and' attenuating'device 4S for the frequency FQ The rectangle 5i) represents a circuit arrangement designed to indicate when the'outputsfrom the attenuatingfdevices 48 .and e9 are equal, in amplitude, .this equality of amplitude being obtainable by adjustment" of theattenuating devices 48and .49.

Figure '5 represents an example of asuitable form ofthe apparatus of Figure 4, using a differential detector.

The detector shown in Figure 5 consists essentially of two diodes 5l and 52. The detector circuit is connected at the points A and Bi respectively to the outputs of the attenuating devices 48 `and 49 of Figure 4 so that the frequency Ft is applied to the anode 53 of the diode 5I whilst the frequency F is applied to the cathode of the diode 52. The direct-current components of the rectified currents pass in opposite directions through the galvanometer 55. 'Io obtain a reading, the attenuating devices 48 and 49 of Figure 4 are adjusted to obtain a null indication of the galvanometer 55, and the difference between the attenuations of the waves F and F0 is deduced from the adjustments of the attenuating devices. A measure of the difference of attenuation in the system under test at the frequencies f and fo respectively is thus obtained.

I claim:

1. In a phase measurement circuit for measuring the displacement of phase during transmission through a quadripole, input terminals and output terminals for connection of a test quadripole, a first carrier oscillator producing a first carrier output of fixed frequency, a second carrier oscillator producing a second carrier output of variable frequency, ya low-frequency oscillator producing a first modulating frequency, a source of a second modulating frequency which is a multiple of said first modulating frequency, a first modulator having carrier input terminals connected to said first carrier oscillator and further having modulating input terminals connected to said low frequency oscillator, a second modulator having carrier input terminals connected to said second carrier oscillator and also having modulating input terminals, an adjustable phase shifting device connected between said source and the modulating input terminals of said second modulator, a mixer unit having input terminals connected to the output terminals of said modulators, and further having output terminals connected to` said input termi- Y nals, a detector having input terminals connected to said output terminals, a first principal band filter adapted to select said rst modulating frequency, a second principal band filter adapted to select said second modulating frequency, the input -terminals of said filters being connected to the output terminals of said detector, a. principal frequency multiplier having input terminals connected to the output terminals of said first band filter and being adapted when so connected to deliver at its output terminals an output of said second modulating frequency, and a principal phase measuring device having a first set of input terminals and a second set of input terminals connected respectively to the output terminals of said principal frequency multiplier and to the output terminals of said second principal band filter and adapted to indicate the phase rel-ations between the two inputs so apits input connected to said input terminala .a

first auxiliary band filter adapted to select said` first modulating frequency, a second auxiliary band filter adapted to select said second modulating frequency, the input terminals of said auxiliary filters being connected to the output terminals of said auxiliary detector, an auxiliary frequency multiplier having input terminals connected to the output terminals of said first auxiliary band filter, and being adapted when so connected to deliver at its output terminals an output of said second modulating frequency, and an auxiliary phase measuring device having a first set of input terminals and a second set of input terminals connected respectively to the output terminals of said auxiliary frequency multiplier and to the output terminals of said second auxiliary band filter and adapted to indicate the phase relations between the two inputs so .applied respectively to its said two sets of input terminals.

4. A phase measurement circuit according to claim 1, said principal phase measuring device comprising a first attenuator having input terminals connected to the output terminals of said principal frequency multiplier and further comprising a second attenuator having input terminals connected to the output terminals of said second principal band filter, a secondary adjustable phase shifting device having input terminals connected to the output terminals of said first attenuator, a tertiary adjustable phase shifting device having input terminals connected to the output terminals of said second attenuator, a cathode ray oscillograph having a vertical set of plates and a horizontal set of plates, the output terminals of said secondary phase shifting device being connected to one said set of plates and the output terminals of said tertiary phase shifting device being connected to the other said set of plates.

5. A phase measurement circuit according to claim 4, and a pair of precision frequency multipliers respectively inserted between each of said secondary and tertiary phase shifting devices and a corresponding said set of plates.

6. An arrangement for the measurement of the difference between the phase delays caused by a quadripole on two frequencies f and fo, cornprising input terminals and output terminals for connection of a test quadripole, a first generator producing a fixed frequency fo modulated by a frequency F0 and a second generator producing a variable frequency f modulated by a frequency F different from F0, means for applying simultaneously these two modulated frequencies to said input terminals, a non-selective detector connected to said Voutput terminals to demodulate said frequencies, two band filters connected to the output of said detector and separating the demodulated .components Fo and F taken off at Y UNITED STATES PATENTS the output of said detector, means for transforming the frequency of the one of the two said com- Nluggela Nyqlsle Oct lstelgzq ponents into a frequency equal to the frequency 20 4.71182 Jensen July' 14 1936 of the other component, and a differential phase- 5 2214130 Green e'a Sept 10 1940 `measuring device having two pairs of inputter- 2;285038 Loughlin Jue 2 1942 minals, one pair of input terminals being con- 2324215 Kinsburgz July 13: 1943 nected to the output of the detector, and the 2337540 Burgess Dea 28, 1943 other pair of input terminals being connected to 3371541 Burgess Dea 28, 1943 the output of said frequency transforming 10 2,364,190 Burgess Dea 5. 1944 means- 2,401,411 Carlisle, Jr. June 4, 1946 JACQUES SELZ.

REFERENCES CITED 

